P700

The reaction center chlorophyll (or the primary electron donor) of photosystem I that is most reactive and best in absorbing light at wavelength of 700 nm.

Supplement

P700 consists of a group of pigments acting as a single entity or like a single molecule when it receives excitation energy. It is named after the wavelength (in nanometers) of their red-peak absorption maximum, which in this case, is the deep-red or 700 nm.